Following the introduction of mandatory Canadian folic acid flour fortification in mid-1997, the incidence of selected childhood cancers that declined in Ontario prior to and subsequent to this public policy initiative was examined. A population-based cohort study of all incident cases of childhood malignancy in Ontario between the years 1985 and 2006 was conducted. Participants were identified from a database provided by the Pediatric Oncology Group of Ontario and included children 0 to 4 years of age and 5 to 9 years of age who were diagnosed with cancer. Among children aged 0 to 4 years, the incidence rate of Wilms' tumor declined from 1.94 to 1.43 per 100,000 (incidence rate ratio 0.74, 95% confidence interval, 0.57-0.95). No significant change was seen in the prefortification vs postfortification time periods for acute lymphoblastic leukemia, brain cancers, or embryonal cancers among the 0- to 4-year or 5- to 9-year age groups. There was an approximately 30% reduction in risk of Wilms' tumor following introduction of the initiative. This corroborates a recent case-control study from Germany. These data may also provide some reassurance that universal flour fortification does not heighten the risk of pediatric cancer.